1. Field of the Invention
The present invention relates to a system for data communication through a communication bus, and more particularly to a system for data communication in a network of components of an audiovisual system on an automobile.
2. Description of the Prior Art
In recent years, automobile audio systems have been changing from systems which give a user only audio information such as music to systems which allow the user to enjoy both audio and visual information. Systems with both audio and visual capabilities are known as audiovisual systems.
Automobile audiovisual systems on automobiles are composed of many different audio and visual components. The audio components include a cassette tape deck, a radio tuner, a CD (compact disk) player, etc., while the visual components include a TV (television) tuner, a navigation system, etc.. Audio signals produced by some of these components are amplified by an amplifier and reproduced from loudspeakers in the automobile. Video signals produced by other components are displayed on a display unit in the automobile. Today, the audiovisual system components are controlled according to digital technology, i.e., by respective controllers in the form of microcomputers.
The components of an audiovisual system are required to be controlled systematically for systematic operation of the components. To meet this requirement, the controllers of the respective components are connected by a communication bus network, so that control data for the components will be transmitted through the communication bus.
In the conventional audiovisual system network, the controllers of the components are controlled by a polling process. According to the polling process, either one of the controllers is given a priority status and handled as a master unit, with the other controllers as slave units, and when the master unit is to collect data from a slave unit, the master unit always accesses the slave unit, i.e., initiates the transfer of the data from the slave unit.
When the master unit transmits communication data to a slave unit for access or the slave unit transmits data back to the master unit, it is necessary that the controllers be identified or specified. For this purpose, the controllers are allotted respective addresses indicating themselves.
The audiovisual system includes a slave unit having a display function in the form of a display unit or the like for the centralized display of various data from slave units or sources such as the CD player, the tuner, the cassette tape deck, etc.
The data are transferred from each source to the display unit in formats that are determined depending on the sources for data compression. One example of such display data transfer format is shown in FIGS. 20, 21A and 21B of the accompanying drawings. That is, the display data includes disk number, playing minute, playing second, track number, playing step (playing order of disk), and two bit data shown in FIGS. 21A and 21B.
The display unit, or the slave unit with the display function, displays the data transferred from another slave unit in a predetermined format. When a new slave unit such as a DAT (Digital Audio Tape recorder) is additionally connected to the audiovisual system, since a format for data to be transmitted from the newly connected slave unit has not been registered in the display unit, the display unit cannot decode and hence display the data from the newly connected slave unit. Consequently, the conventional audiovisual system or network lacks an ability to expand itself, i.e., expansibility.